The gunman who shot Rep. Gabrielle Giffords in the head and then unleashed a spray of bullets that killed six and wounded 12 others at a town hall event outside a Tucson, Ariz., grocery store may not have acted alone, authorities announced Saturday night.

"We are not convinced that [the gunman in custody] acted alone, there is some reason to believe he came to this location with another individual, and that individual is involved," Pima County Sheriff Clarence Dupnik said.

Police said a suspect was taken into custody, and Dupnik described the alleged shooter as mentally unstable. Though the sheriff did not name the suspect, he was identified by multiple law enforcement sources as Jared Lee Loughner, 22.

Dupnik declined to provide more information on the second individual who he would only describe as "white" and "in his 50s." Authorities have photographs of the person of interest and are "actively pursuing him," the sheriff said.

The congresswoman was the clear focus of the gunman's assault, Dupnik said.

"He ran through the crowd and when he got to [Giffords] he just started shooting," the sheriff said.

"The anger, the hatred, the bigotry that goes on in this country is getting to be outrageous. And unfortunately Arizona I think is the capital. We are the Mecca for prejudice for prejudice and bigotry," he said.

Dupnik said authorities are also investigating a suspicious package at one of Giffords' field offices in Tucson.

Steven Rayle, who was at the scene, told ABC News that "another man went running off" after the shooting, but wasn't sure if he was involved.

A total of 19 people were hit by gunfire during the shooting, which began just after 10 a.m. Six of the victims have died, according to Dupnik, and the remaining individuals are not believed to be in "life threatening situations."

Five of the victims died on the scene, and 9-year-old Christina Greene died at a nearby hospital.

Those who died and have been identified are 63-year-old U.S. district Judge John Roll , 30-year-old Gabriel Zimmerman (Giffords' director of community outreach), 76-year-old Dorthy Murray, 76-year-old Dorwin Stoddard and 79-year-old Phyllis Scheck, according to the Pima County Sheriff's Department.

Giffords, 40, was shot in the head and was taken to University Medical Center, where she underwent brain surgery and was listed in critical condition.

Dr. Peter Rhee, trauma director at UMC, told reporters that the bullet went through her brain, but he said she was responsive to doctors' commands.

"I am very optimistic about her recovery," Rhee said.

Richard Carmonia, a former U.S. surgeon general who was at the event where the shooting occured, said that Giffords' injuries were "severe" but that there is a "possibility she will survive."

President Barack Obama, speaking at a nationally televised news conference, called the shooting an unspeakable tragedy.

"What Americans do at times of tragedy is to come together and support each other, so at this time I ask all Americans to join me and Michelle in keeping all the victims and their families, including Gabby, in our thoughts and prayers," he said. "We are going to get to the bottom of this and we're going to get through this."

"The suspect is currently in custody, but we don't yet know what provoked this unspeakable act," the president said.

FBI Director Robert Mueller was headed to Arizona to coordinate the investigation, Obama said. Federal authorities have jurisdiction in an attack on a sitting member of Congress.

Giffords' husband, astronaut Mark Kelly, was en route to Tucson from Houston, where he has been preparing to command the last scheduled space shuttle mission, which is supposed to launch in April.

Dupnik said he had no reports on what Loughner may have said during the shooting but said that he would describe him as "unstable."

The Pima County Sheriff's Office is taking the lead of the investigation and was assisted by the FBI, Joint Counterterrorism Task Force and the U.S. Capitol Police.

An urgent email message to members of Congress from the Capitol Police informed them of the shooting and advised them to "take reasonable and prudent precautions regarding their personal security at home and in public forums."

Giffords, a Democrat, was holding a "Congress on Your Corner" event at a Safeway supermarket in northwest Tucson when the shooting occurred.

Rayle, who was at the event, told ABC News a man approached Giffords and shot her before turning his gun on other people randomly.

"I went to the side of the table by a concrete post and I looked up and saw a man shoot her in the head. And then he began spraying gunfire everywhere. At that point I ducked behind a concrete post and as he came around it," he said. "The whole thing unfolded in about 12 or 15 seconds. As he came around it I laid down on the ground and acted as if I was shot."

Rayle, who eventually helped subdue the shooter, said the gunman had an "impassive face" as he was shooting.

"He did try to escape and I think one of her staffers tackled him," Rayle said. "I assisted in holding him down until more people arrived. He did struggle. He did not say anything."

Jason Pekau, another witness who worked at a nearby Sprint store, said he heard 15 to 20 gunshots and saw lots of people running and screaming that the Congresswoman had been shot.

Pekau also said two bodies were covered on the sidewalk after emergency responders had arrived at the scene.

A Democratic member of Congress who asked not to be identified said one of the dead was a Giffords staffer.

Washington was in shock after the shooting and statements condemning the violence poured in from both sides of the political aisle.

"Whoever did this; whatever their reason, they are a disgrace to Arizona, this country and the human race, and they deserve and will receive the contempt of all decent people and the strongest punishment of the law," Sen. John McCain of Arizona said in a statement.

House Speaker John Boehner said he was "horrified" and strongly condemned the attack.

"An attack on one who serves is an attack on all who serve," he said in a statement.

Democratic Leader Nancy Pelosi called the shooting a "national tragedy" and praised Giffords as a "brilliant and courageous member of Congress."

Giffords Had Been Target of Vandals

Giffords, a representative for Arizona's 8th District who just won reelection to a third term, has been the target of conservative political opponents in recent months.

In March of last year, Giffords' office was vandalized just after she voted in favor of the health care reform law. The intruders destroyed a glass door and a window at her Tucson Congressional offices.

At the time, Giffords' press secretary C.J. Karamargin said the office had received many phone calls with "nasty and rude and hateful comments" from opponents of the health care bill.

Recently Giffords, who supports gun rights, has been receiving angry letters from anonymous sources, ranting about the supposed national gun registry and border control.

One letter received on Dec. 15 addressed to "giffords, obama, mccain and sen. Jon kyl" got personal on the topic of U.S. Border Patrol Agent Brian A. Terry, who was killed in a late night shootout at the U.S.-Mexican border.

"Brian a Terry's blood is on your hands! How many more legal residents of the USA have to die before the border is CLOSED??? Obama I call you out! CLOSE THE DAMN BORDER NOW! Quit pandering to illegals!," the letter read.

Giffords has a reputation as a political rebel. She voted with the Democratic party about 40 percent of the time, according to Congressional Quarterly, though she supported her party's effort to pass a landmark health reform law.

She was also one of a handful of Democrats who did not vote for Nancy Pelosi for speaker of the house, during the recent Democratic leadership elections after Democrats lost their majority in the House last November.

At a candlelight vigil Saturday night, authorities found a suspicious package outside Giffords' office and later detemined it was non-explosive, the Associated Press reported.

Police said the item looked like a coffee can and contained writing which was not disclosed to the media, according to the Associated Press.